1. Technical Field
This invention relates to a device for measuring the size of a target located in the field of view of a videoendoscopic probe.
It is in particular, but not exclusively, applicable to industrial endoscopy.
The terms “endoscope” or “fiberscope” refer to a rigid or flexible probe intended to be inserted into a dark cavity and enabling the user to see through an eyepiece the image of a target located in the cavity. Such a probe therefore includes a target illuminator and an optical device providing the user with an image of the target. The optical device includes a distal objective, an image transport device that is rigid and consists of a series of lenses or flexible and consists of an ordered fiber optic bundle, and a proximal eyepiece in which the user can see the image of the target. The illumination device generally includes an illuminating fiber bundle of which the distal end, suitably arranged near the distal objective, illuminates the target when its proximal end is connected to a light generator.
The term “videoendoscope” refers to a flexible or rigid probe enabling the user to see on a video screen the image of a target located in a dark cavity. Therefore, a videoendoscope includes a target illuminator identical to that of an endoscope or a fiberscope and an optoelectronic device providing the user with a video image of the target. A videoendoscope can be produced by connecting the eyepiece of an endoscope or a fiberscope to the objective of an endoscopy camera, or can be specifically designed with a videoendoscopic probe, and include:
a distal endpiece housing an optoelectronic device including in particular a CCD sensor having a photosensitive surface on which the image, provided by the objective with which it is associated, is formed.
an inspection tube, which is most often flexible, of which the distal end is rigidly connected to the distal endpiece,
a control handle rigidly connected to the proximal end of the inspection tube,
a flexible umbilical connecting tube of which the distal end is rigidly connected to the control handle and the proximal end is intended to be connected to an external enclosure containing in particular a light generator and an electrical power source,
an illumination fiber bundle housed in the umbilical tube, in the control handle, then in the inspection tube, and of which the distal end, housed in the distal endpiece, illuminates the target when its proximal end is connected to a light generator,
a video processor transforming, into a useful signal, the electrical signal provided by the distal CCD sensor to which it is connected by a multiconductor power cable, and of which the synchronization is adjusted according to the length of the cable,
a control panel enabling the operation of the video processor to be adjusted in particular according to the color temperature of the lighting of the target by the distal end of the illuminating fiber bundle of the videoendoscopic probe, and
a video monitor connected to the video processor and implanted, preferably adjacent to the control handle.
The videoendoscopic probes can also have the following functions:
a distal hinged tip deflection enabling the direction of the distal endpiece of the probe to be adjusted, with the control handle including mechanical or electromechanical control means for actuating said tip deflection,
interchangeable optical heads that can be fitted on the distal endpiece of the inspection tube and enabling the optical field covered by the videoendoscope and/or the directions of the videoendoscope illumination and optical axes to be modified, and
a digital image freeze device for recording, mapping and processing images, which can be either a simple laptop computer with a video input, or a dedicated system controlled by the videoendoscope control panel which will preferably be located on the control handle.
As regards the inspection of mechanical parts, it may be desirable to complement the videoendoscopic probe display function with a metrology function enabling the user to directly measure the size of certain elements of a target being inspected. The integration of a metrology function in a videoendoscopic probe involves the implementation of a measuring method and measuring devices specific to this method.
The measuring devices used in videoendoscopy generally include the following two means:
an optical means integrated in the distal end of the videoendoscopic probe, for inserting an auxiliary image in the image of the target displayed on the video screen connected to the probe, this auxiliary image characterizing the real position of the target in the optical field covered by the probe, and
a digital image processing means for enabling the user to map the ends of the video images of the target and the auxiliary image displayed on the video screen connected to the probe, and then for applying a computational algorithm enabling the real size of the target to be deduced from the mapping.
The implementation of the distal optical means specific to the chosen measuring method in a videoendoscopic probe should preferably involve the integration of this optical means in a distal measuring head that can be removed and interchanged with the conventional optical heads of the probe. It should be noted that if a removable measuring head is used, the mechanical devices enabling the head to be positioned and locked on the distal end of the videoendoscopic probe must satisfy strict precision criteria, while meeting the following more general requirements:
the establishment of continuity between the optical paths and the illumination paths of the videoendoscopic probe and the removable head, which requires the simultaneous implementation of a longitudinal lock and side indexing,
the prevention of any possibility of accidental unlocking of the removable head, and
the absence of CCD sensor pollution by parasitic light rays emanating from the probe's illumination path.
The mechanical means implemented to meet these requirements vary depending on the optical structure of the probe/removable head pair. Most often, as is the case in U.S. Pat. No. 4,727,859, the CCD sensor of the probe is securely attached to a distal optical device having a smaller frontal overall dimension than the CCD sensor. Under these conditions, this optical device can be housed in the distal portion of the probe which has a diameter smaller than that of the probe itself, so that it can be inserted into the proximal tubular end of removable heads having a diameter identical to that of the probe. This architecture has the advantage of simplifying the locking devices and the disadvantage of requiring another optical device in addition to that integrated in the probe to be housed in the removable heads. To reduce the overall length of the optical system, it would be more technically advantageous to house the entire optical system in the removable heads which are attached directly to the CCD sensor integrated in the probe, taking into account that, under these conditions, the mechanical locking devices are more delicate to produce.
2. Description of the Prior Art
The measuring methods used in videoendoscopy can be grouped into two categories, namely:
so-called direct measuring methods which involve simultaneously displaying, on the video screen of the videoendoscopic probe, an image of a known size reference associated with the target, which methods, under these conditions, involve mapping the ends of the image of the target and the image of the reference and applying a simple comparison algorithm enabling the real size of the target to be deduced from the mapping, and
so-called indirect measuring methods which involve simultaneously displaying, on the video screen of the videoendoscopic probe, the image of the target of unknown size which is to be measured, and the image of an auxiliary element associated with the target, whose position on the video screen reflects a significant physical parameter of the measurement, such as, for example, the observation distance separating the target from the distal end of the videoendoscopic probe, which methods involve, under these conditions, mapping the image of the auxiliary element and the ends of the image of the target and applying an algorithm in order to deduce, from the mapping, the observation distance, the real coordinates in space of the ends of the target, and finally the real size of the target.
Various measuring methods used in videoendoscopy are briefly described below.
Direct Measurement by Comparison of Two Elements of the Target
This method involves simultaneously displaying two elements of the target located near one another, one of unknown size which is to be measured and the other of known size, then directly comparing the size of the video image of the unknown element with the size of the video image of the known element. This method, which is very simple to implement, cannot be used universally insofar as it is unusual to have an endoscopic image showing a reference element of known size located next to the target to be measured.
Direct Measurement by Gridding the Image of the Target
This method of modeling, described in patent JP 11045349, involves imbedding, by electronic means, a “three-dimensional” gridded network specific to the piece to be inspected into the video image provided by the videoendoscopic probe, then deducing the size of a target located on the piece, from the size of the grid in which the video image of said target is located. This method, which is very difficult to implement, is only of very limited practical interest.
Direct Measurement by Projection of a Collimated Laser Beam onto the Target
This method, which is described in patent GB 1 573 142, involves projecting in the viewing field of a video camera, a cylindrical collimated laser beam parallel to the optical axis of the camera so as to form near the target a circular reference light spot of known and unvarying size. The measurement of the size of the target involves directly comparing, on the video screen associated with the camera, the unknown size of the image of the target with the known size of the image of the reference light spot. This method, which was described in patent GB 2 269 453 in reference to an endoscope having an integrated laser fiber, is perfectly adaptable to videoendoscopy. Unfortunately, the small size of a collimator that can be integrated in the distal end of a videoendoscopic probe with a small diameter drastically reduces the diameter of the collimated laser beam. This limitation causes, from a relatively short observation distance, the generation of a video image of the laser spot having such a small diameter that any measurement by comparison would quickly become inaccurate.
U.S. Pat. No. 4,281,931 describes a variant of this direct measuring method, involving an annular collimated laser beam generated at the distal end of a laser fiber integrated in a side-view fiberscope.
Another variant of this method was used in a fiberscope described in patent DE 36 29 435 with two laser fibers integrated in the fiberscope and projecting, in the field of view, two laser beams parallel to the viewing axis so as to form, near a target located in the field of view, two light points separated by a known and unvarying distance. The measurement of the size of the target involves directly comparing, on the video screen associated with a camera connected to the eyepiece of the fiberscope, the unknown size of the image of the target with the known distance separating the images of the two light points.
The endoscopes of the current prior art in the field, implementing the direct measuring method described above, have the following limitations:                the transmission by an optical fiber of the laser beam generated by a proximal laser source, requires a proximal optical matching device which leads to losses, and a distal optical device for collimation of the light beam, of which the performance is limited by the size of the endoscope's distal endpiece, and        the adaptation to the conditions for viewing of the optical field observed and the shape of the projected laser beam is impossible.        
Indirect Measurement by Projection of a Laser Beam on the Target
This method, which is described in patent FR 2 630 538, involves projecting in the observation field of a video camera, a laser beam with an axis parallel to the optical axis of the camera and generated at a known distance from the optical axis, so as to form a light point near the target. The measurement of the real coordinates of the ends of the target are deduced by mapping, on the video screen associated with the camera, the image of the laser point and the ends of the image of the target. Moreover, this patent explicitly mentions the implementation of this method in endoscopy.
The inaccuracies of mapping the center of the video image of the laser point due to the absence of any collimation device associated with the distal end of the laser fiber integrated in a probe make the implementation of this videoendoscopy method more delicate than the direct measuring method described in the preceding paragraph. Nevertheless, this indirect measuring method is commonly used to naturally obtain observation distance measurements enabling the distal end of an endoscope to be positioned almost automatically at a predetermined distance from a target.
A variant of this method has thus been implemented in a fiberscope described in patent DE 36 29 435, using a laser fiber integrated in the fiberscope and projecting in the field of view a laser beam inclined on the viewing axis of the fiberscope so as to form on the target a light point generating an image of which the separation from the center of the image field is dependent on the observation distance.
A similar variant of this method has also been implemented in an endoscope described in patent FR 2 480 107, using a laser fiber integrated in the fiberscope and projecting in the field of vision a laser beam inclined on the viewing axis of the endoscope so as to form on the target a light point generating an image of which the coincidence with the center of the image field characterizes the mapping distance.
The endoscopes of the current prior art in the field, implementing the indirect measuring method described above, have the following limiting features:
the transmission by an optical fiber of the laser beam generated by a proximal laser source, requires a proximal optical matching device leading to losses, and
the adaptation to the conditions for viewing of the optical field observed and the shape of the projected laser beam is impossible.
Indirect Measurement by Projection of an Auxiliary Image onto the Target
This method, which is described in patents DE 28 47 561 and U.S. Pat. No. 4,660,982, involves projecting onto the target viewed by an endoscope an auxiliary image generated by a mask associated with an objective integrated in the distal end of the endoscope's illumination device. The observation distance and the size of the target can be derived from the positioning and size in the image field of the endoscope's eyepiece, of the image of the target and of the image of the auxiliary image projected onto the target.
Although it is commonly used in videoendoscopy according to the implementation methods described in patents U.S. Pat. Nos. 4,980,763 and 5,663,675, this method has serious limitations concerning the depth of the field of measurement, due to the fact that the auxiliary image projected onto the target is clear only at a given observation distance. Beyond this observation distance, the video image of this auxiliary image is substantially blurry, which leads to imprecision in the mapping of this video image, as well as in the measurements derived therefrom.
Indirect Measurement by Image Doubling
This method involves forming on the sensitive surface of the distal CCD sensor of a videoendoscopic probe, two images of the target seen from different angles using two separate distal optical paths. The observation distance and the size of the target can then be derived using electronic tools for mapping and computing the relative positions and sizes of these two images on the probe video screen.
These two images can be simultaneously generated by two separate objectives placed in the distal end of the probe (U.S. Pat. No. 4,873,572, US 2002/0137986, U.S. Pat. No. 6,063,023), or sequentially, owing to the alternate use of two aperture plates integrated in the distal objective of the probe and arranged symmetrically with respect to the optical axis (U.S. Pat. No. 5,222,477). The two methods mentioned above have serious problems concerning the integration of the required optical means in the distal end of a videoendoscopic probe with a small diameter.
Already mentioned in patents DE 34 32 583 and DE 41 02 614, a variant of this method which is more suitable for videoendoscopy was described in patent U.S. Pat. No. 6,411,327. This variant involves placing in front of the distal end of the objective of a video camera, an image doubling device consisting of a single one-piece optical component having a planar distal surface and a proximal surface in the shape of a delta with a projecting edge. The limitations, both in width and in depth, of the optical field of measurement specific to this type of device unfortunately restrict the scope of use of this appealing method.